Antireflector black matrix for display devices comprising three layers of zinc oxide, molybdenum, and zinc oxide

ABSTRACT

A black matrix structure suitable for use in an active matrix liquid crystal display is described. The novel antireflective black matrix comprises successive layers of zinc oxide, molybdenum, a second layer of zinc oxide and a molybdenum mirror, all of which are deposited upon the front surface of a glass substrate member. An alternative structure includes the same sequence of layering on the back surface of the molybdenum mirror. The described black matrixes reduce reflectance of incident light to less than 0.5%. A technique for the preparation of the described structure involving cathodic sputtering is also described.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. application Ser. No.08/351,969 filed Dec. 8, 1994, which is hereby incorporated herein byreference.

FIELD OF THE INVENTION

This invention relates to liquid crystal display devices. Moreparticularly, the present invention relates to active matrix liquidcrystal displays including an antireflective black matrix and to atechnique for the preparation thereof.

BACKGROUND OF THE INVENTION

During the past two decades, there has been a birth of interest in aclass of devices commonly referred to as electroluminescent displayswhich are solid state devices that use the direct production of light inthin films to provide displays of satisfactory appearance at moderatecost.

This technology led to the development of liquid crystal displays basedupon a phenomenon known as dynamic scattering wherein current is passedthrough a nematic liquid crystal, so causing the material to break upinto domains having randomly directed axes. The next stage in thedevelopment of such devices involved the fabrication of a twistednematic display in which a nematic liquid crystal is disposedintermediate two closely spaced glass plates coated with a polymer andrubbed such that the nematic liquid crystal is aligned parallel to therubbing direction. If the plates are robbed at 90 degree angles, theliquid crystal deforms into a twisted structure. Passage of polarizedlight through the display results in the plane of polarization followingthe twist. The application of an electric field to the display permitsthe liquid crystals to rotate and align themselves with the field,thereby leading to disruption of the twist and rotation of polarization.This permits either full transmission or no transmission in the display.

Later developments in the field led to the generation of thin filmtransistor/liquid crystal displays and to color liquid crystal displays.In the case of the latter, workers in the art have recognized that areduction in efficiency is ofttimes encountered by absorption of lightin the color filters. Accordingly, efforts have continued to alleviateor eliminate this limitation.

Heretofore, the most common technique for overcoming this problem hasbeen to design an active matrix liquid crystal display including a glasssubstrate (front glass plate) having a black matrix typically comprisingchromium oxide deposited upon a portion thereof. The purpose of thismatrix is to define and outline individual pixels which is of particularimportance for color displays in providing a separator between theelements of the color filter from each other.

Additionally, the black matrix serves to mask the electronics on theback plate of the device, thereby providing protection therefor fromambient light and associated photodegradation.

Black matrix structures employed heretofore typically comprise a grid ofblack lines, usually of the order of 20 nanometers in thickness spacedso as to provide an array of open opaque rectangles, typically 100 μm ona side. The black matrix is also chosen to minimize reflection of lightwhich tends to result in the degradation of the display.

In the past, numerous approaches for attaining this end have beenemployed by those skilled in the art. Thus, for example, black matrixeshave included grids made from (a) dyed black photoresists, (b)photoresists impregnated with carbon black, (c) patterned chromium, (d)patterned chromium/chromium oxide, and (e) patterned chromium/chromiumoxide/chromium trilayers. Both the bilayer and the trilayer structureshave been engineered to minimize reflectance of the matrix. Studies haverevealed that the best of the matrixes among these prior art structuresevidences an average reflectance of approximately 3%. Accordingly,efforts have continued in the search to enhance this characteristic.

SUMMARY OF THE INVENTION

In accordance with the present invention, this end has been attained bythe fabrication of a novel black matrix structure which has evidencedthe ability to reduce reflectance of incident light to less than 0.5%.

The structure of interest is constructed of four layers depositedsuccessively upon the back side of a suitable front glass substratemember, the layers being comprised of zinc oxide, molybdenum, zinc oxideand a molybdenum mirror. Alternatively, the structure may be doublesided and include a black matrix of the same type on the back side ofthe molybdenum mirror employed herein, so forming a seven layerstructure.

BRIEF DESCRIPTION OF THE DRAWINGS.

The invention will be more readily understood by reference to thefollowing detailed description taken in conjunction with theaccompanying drawing wherein:

FIG. 1 is a front elevational view in cross-section of a four layerblack matrix structure prepared in accordance with the presentinvention;

FIG. 2 is a front elevational view in cross section of a seven layerblack matrix structure prepared in accordance with the invention; and

FIG. 3 is a front elevational view in cross section of a color filterincluding the black matrix of the invention as destined for use in athin film transistor liquid crystal display.

DETAILED DESCRIPTION OF THE INVENTION

The first step in the practice of the present invention involvesselection of a suitable substrate for use in the liquid crystal displayto be prepared. For this purpose, it is common practice to employ aconventional glass plate obtained from commercial sources. The glassplate so chosen is then placed within a cathodic sputtering chamberhaving a zinc target and reactive sputtering effected in the presence ofoxygen to yield a first zinc oxide layer on the substrate having athickness ranging from 35-60 nanometers. Alternatively zinc oxide may bedeposited by sputtering from a zinc oxide target.

Thereafter, the oxygen flow in the chamber is terminated and the zinctarget either kept in place or replaced by a molybdenum target. Then,sputtering is effected to yield a molybdenum coating upon the first zincoxide coating having a thickness ranging from 6 to 11 nanometers. Thethickness of the metal film is dictated by considerations relating tominimizing the reflectance of the front face of the entire structure.

Next, the zinc target is again inserted in the sputtering chamber (ifpreviously removed) and reactive sputtering in the presence of oxygenagain effected to yield a second zinc oxide layer upon the metal layer,said second zinc oxide layer having a thickness ranging from 35-50nanometers. Thickness of the second zinc oxide layer is also determinedby considerations relating to minimizing the reflectance of the frontface of the entire structure.

Then, a molybdenum mirror is deposited upon the second zinc oxide layerby cathodic sputtering of molybdenum in the sputtering chamber employedin the practice of the invention to yield the desired four layer blackmatrix which has been found to reduce the reflectance of incident lightto less than 0.5%. The black matrix structure is then removed from thesputtering chamber and patterning effected by conventionalphotolithographic techniques including etching to define the desiredpixel windows. Studies have revealed that the thicknesses of the threemiddle layers of the black matrix structure of the invention areinterdependent and critical. It will be noted that average reflectancesin each case are significantly lower than those reported for similarprior art structures.

In Table I set forth below, a detailed analysis of the characteristicsof a black matrix structure in accordance with the present invention isshown.

                  TABLE I                                                         ______________________________________                                        Mo mirror/ZnO/Mo/ZnO Antireflector                                            ZnO 1    Mo            ZnO 2   R                                              ______________________________________                                        46.69    5             50.72   1.8                                            46.69    6.25          50.72   0.3                                            46.69    7.37          50.72   0.06                                           46.69    8.75          50.72   0.6                                            46.69    10            50.72   1.7                                            40       7.37          50.72   0.5                                            46.69    7.37          50.72   0.06                                           50       7.37          50.72   0.15                                           46.69    7.37          40      0.8                                            46.69    7.37          50.72   0.06                                           46.69    7.37          55      0.2                                            46.69    7.37          60      0.6                                            ______________________________________                                         Mo mirror: 100 nm thick                                                       R = average relfectance between 40 nm and 700 nm film thickness in nm         ZnO 1 is adjacent to mirror; ZnO 2 is adjacent to glass                  

The Table reveals that although layer thicknesses are critical to theperformance of the multilayer stack, variations may be made from theoptimum values to yield a structure evidencing satisfactory lowreflectance characteristics. This is of particular significance in thefour layer structure and is a decided advantage over three layerstructures which have more limited tolerances.

It should be noted that molybdenum and zinc oxide are of particularsignificance due to etching considerations. As noted previously, themost efficient procedure for fabricating the black matrix of theinvention involves depositing films of the constituent components overthe entirety of the black surfaces of a front glass substrate of thedesired device and then etching out pixel windows. Molybdenum and zincoxide are uniquely suited for this approach since zinc oxide andmolybdenum can both be etched by the same etchant, so assuring thatpatterning of the black matrix can be effected in one photolithographicetching step, thereby assuring further economies in the preparationthereof.

With reference now to FIG. 1, there is shown a front elevational view incross-section of a typical black matrix structure in accordance with theinvention. Shown in the figure is a glass substrate 11 having depositedthereon successively on the back side thereof a first layer of zincoxide 12, a molybdenum layer 13, a second layer of zinc oxide 14 andmolybdenum mirror 15.

A structure equal in use to that shown in FIG. 1 may be prepared bydepositing the same antireflective coating on the back side of thechromium mirror of the four layer stack of FIG. 1. Such a structure isshown in FIG. 2 and this structure comprises a first zinc oxide layer16, a second molybdenum layer 17 and a second zinc oxide layer, 18. Thisdouble-sided seven layer structure is antireflective with respect bothto ambient light incident from outside the display device and to lightfrom the backlight that is integral to the device, light which otherwisewould be reflected back into the display electronics.

With reference now to FIG. 3, there is shown a front elevational view incross-section of a color filter including the black matrix of theinvention as depicted in FIG. 1 or FIG. 2 herein. The color filter shownis suitable for use in a thin film transistor liquid crystal display.

Shown in FIG. 3 is a glass substrate 21 having deposited thereon apattern of black matrix structures 22 prepared as described herein byphotolithographic techniques. Matrixes 22 serve as spacers between colorfilter elements 23, 24 and 25 which are typically, red, green and bluefilters, respectively. Deposited upon the top surface of the structureis an electrode, for example, indium tin oxide.

Based upon the data set forth in Table I, it is evident that the blackmatrixes prepared in accordance with the described technique evidencesuperior antireflective characteristics to those of the prior art andoffer workers in the art a viable conduit to commercial applications.

While the invention has been described in detail in the foregoingdescription, it will be appreciated by those skilled in the art thatmany variations may be made without departing from the spirit and scopeof the invention. Thus, for example, zirconium and titanium may beemployed as the metal member of the black matrix structure. However, itwill also be understood by those skilled in the art that suchsubstitution will necessitate alternate means for removal thereof inunwanted areas of the glass substrate since these materials cannot beetched by standard photolithographic techniques. It will also beunderstood by those skilled in the art, that during the sputteringprocess, variations may occur in the nature of the product deposited.

The invention claimed is:
 1. An active liquid crystal display includingan antireflective black matrix deposited upon the back side of a glasssubstrate member comprising successively a first zinc oxide layer with athickness in the range of 6-11 nanometers, a molybdenum layer with athickness in the range of 35-60 nanometers, and a second zinc oxidelayer with a thickness in the range of 35-50 nanometers and a molybdenummirror.
 2. Display in accordance with claim 1 further comprising a firstzinc oxide layer, a molybdenum layer and a second zinc oxide layer onthe back surface of said molybdenum mirror.
 3. Method for thefabrication of an antireflective black matrix for use in a thin filmtransistor liquid crystal display which comprises the steps of(a)depositing a first layer of zinc oxide upon the back side of a frontglass substrate in a thickness ranging from 35-60 nanometers; (b)depositing upon said first layer a layer of molybdenum in a thicknessranging from 6-11 nanometers, (c) depositing a second zinc oxide layerupon the molybdenum layer in a thickness ranging from 35-50 nanometers;and (d) depositing a molybdenum mirror upon said second zinc oxidelayer.
 4. The method of claim 3 wherein at least one layer of zinc oxideis deposited by reactive sputtering of zinc in the presence of oxygen.5. The method of claim 3 wherein said layer of molydbenum is depositedby cathodic sputtering.
 6. The method of claim 3 wherein at least onelayer of zinc oxide is deposited by cathodic sputtering of zinc oxide.7. Display in accordance with claim 1 wherein said antireflective blackmatrix reduces the reflectance of incident light to less than 0.5%.